Date of Award

Summer 2019

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Enrica Viparelli

Abstract

Research efforts on mixed bedrock-alluvial rivers primary focused on the bedrock incision and very few studies investigated the alluvial morphodynamics of such systems. To the best of my knowledge none of these models have been considered the spatial variability of the sediment grain size of the bed surface in mixed bedrock-alluvial reaches and very few models focused on the spatial changes in alluvial cover within these reaches. Furthermore, a perusal of the literature on mixed bedrock-alluvial river morphodynamics reveals that very little information is available on 1) bedform geometry and flow resistances and 2) sediment sorting patterns in presence of a non-erodible bedrock surface. Understanding the interactions between flow and sediment transport processes in mixed bedrock-alluvial reaches is important to e.g. predict the long-term river response to engineering works, changes in climate and sediment supply; perform large scale sediment budgets; and determine the quality of the riparian habitat. I thus designed and performed laboratory experiments to investigate the effects of a model bedrock surface on flow hydraulics and sediment transport processes. I derived a novel mathematical formulation of mixed bedrock-alluvial morphodynamics that accounts for the non-uniformity of the bed material. I implemented this formulation in a one- dimensional model of river morphodynamics. The experiments revealed that equilibrium in mixed bedrock-alluvial reaches is characterized by flow acceleration in the streamwise direction when the slope of the bedrock surface is milder than the equilibrium slope of an alluvial reach transporting the same discharge and sediment load. The morphodynamic response to this spatial flow acceleration is characterized by 1) streamwise reduction in the alluvial cover, 2) streamwise reduction in bedform height, and 3) formation of a pattern of downstream fining of the bed surface sediment. The morphodynamic model was validated at laboratory scale against the experimental results. The validated model was then used to study the changes in flow hydraulics and sediment transport processes in mixed bedrock-alluvial reaches with a bedrock surface slope that was steeper than the alluvial equilibrium slope of a channel subjected to the same discharge and sediment supply of the mixed bedrock-alluvial reach of interest. The numerical results at equilibrium show that in this case flow velocity decreased on the mixed bedrock-alluvial reach in streamwise direction. The morphodynamic effects of this spatial flow deceleration were 1) a streamwise increase in alluvial cover, and 2) the formation of a pattern of downstream coarsening of the bed surface sediment. The morphodynamic formulation presented in this dissertation will be applied at field scale on the gravel bed Buech River, Southeastern France, to study the impacts of dam construction and gravel mining on a mixed bedrock-alluvial gravel bed river, and to identify possible restoration strategies to control the observed widespread erosion and the associated deterioration of the aquatic and riparian habitat.

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